a) Field of the Invention
The invention concerns a capacitor microphone comprising a microphone housing having a sound inlet opening, a diaphragm and a counterpart electrode which is associated with the diaphragm and which is arranged at a small spacing relative to the diaphragm. The invention further concerns a corresponding process for the production of such a capacitor microphone.
b) Description of the Related Art
Several hundred million miniature capacitor microphones are produced yearly worldwide. In general those microphones are produced using stacking technology. The individual elements of the transducer which is used in that case, that is to say in particular a diaphragm ring with a diaphragm glued thereto, a spacer ring, the counterpart electrode and so forth are in that case simply stacked one upon the other in the microphone housing. Such a structure is admittedly particularly simple but it also suffers from deficiencies which make use thereof practically impossible for the production of high-grade microphones and particularly high-grade miniature microphones.
Firstly, stacking technology involves relatively high levels of scatter in terms of the electro-acoustic parameters. The permitted deviations in sensitivity and the frequency response from the reference value and the reference curve are generally in the region of ±3 dB and higher. Experience shows that, even with those generous tolerances, it is not possible to avoid rejects. As the result can only be detected after the capsules (that is to say the microphones) have already been assembled (generally flanged), the parts of the reject capsules can no longer be used. Not only the wage costs but also additional material costs are loaded on to the end product in that case. One of the most important causes of the scatter in respect of sensitivity and frequency responses is the unevenness of the individual parts. That concerns in particular the inside surface of the microphone housing, the diaphragm ring and the electret surface which serves as a reference surface for the air gap between the diaphragm and the counterpart electrode. Diaphragm stiffness is changed due to mechanical deformation of the diaphragm ring in the operation of assembling the capsule, and that in turn causes changes in the electro-acoustic parameters.
Secondly, the capsule in question has a very high stray capacitance which is formed by the capacitances between the counterpart electrode and the diaphragm ring and between the counterpart electrode and the microphone housing. In miniature microphones with a very small effective diaphragm area the stray capacitance gives rise to losses of 3-6 dB in sensitivity.
Thirdly the spacer ring of plastic film often has a burr. That is the cause of the air gap no longer corresponding to its nominal value.
Fourthly the use of the diaphragm ring leads to a reduction in the oscillatable diaphragm area. Thus the oscillatable diaphragm area in miniature microphones frequently constitutes only half the cross-sectional area of the capsule, which gives rise to considerable losses in the dynamic range of the microphone. US No 2002/01-54790 A1 discloses a capacitor microphone in which the diaphragm is adhesively fixed to the underside of a holding ring provided with a sound inlet opening. There, the ratio of oscillatable area of the diaphragm to the total cross-sectional area of the capacitor microphone (assuming a thin housing outer wall in the region of 0.1 mm) is (1.9/2.5)2=0.762=0.57.
DE 3616638 C2, DE 10064359 A1, DE 3852156 T2, DE 2445687 B2 and DD 72 035 also disclose capacitor microphones in which the diaphragm is fixed to a part of the microphone housing. particular that the way in which the diaphragm is fixed to the microphone housing has an influence on the width of the air gap between the diaphragm and the counterpart electrode, which however should maintain a value which is as accurate as possible. Thus for example when fixing by means of an adhesive is involved, it is scarcely possible to set exact flatness of the diaphragm and an air gap between the diaphragm and the counterpart electrode of an exact width, by virtue of the thickness of the adhesive layer, which cannot be exactly predicted.